Overhead sliding door operators presently on the market consist of two basic systems.
(1) They have a load bearing system which supports the weight of the door panel or panels and allows movement of the door. This is normally accomplished by using a precision shaft and linear bearings, or rollers, supported in a C-shaped, bent or extruded, member. Doors are generally supported by door hangers which are free to move along horizontal rods or tracks.
(2) Door operators also have mechanisms for moving the doors. A belt and pulley system, for example, may be used to move the doors. Alternatively, overhead door operators may employ a helical drive member which engages a drive nut attached to a door hanger. A motor is used to rotate the helical drive and this causes movement of the nut along the drive.
These two systems are normally placed parallel to each other, necessitating a linkage between the two systems. For a system employing a helical drive member, the linkage connects the drive nut to the door hanger. This linkage presents design problems because it must transmit a drive force to the door panel and it must accommodate misalignment between the two systems.
Systems presently on the market are also open to dust, dirt, metal chips etc, causing contamination of the screw mechanism and/or of the linear bearings.
In many systems, a 1" diameter precision shaft is used to support the door panels. This limits the available span between supports. Generally, full linear bearings are preferred over split bearings.
For application to a transit vehicle, the two systems may be separately attached to the vehicle or the system may include a base to which the two systems are attached. The base is attached to the vehicle.